The invention relates to the processing of semiconductor devices and is specifically related to plasma etching wherein a CF.sub.4 plasma is employed to generate reactive ions that can attack silicon. Reference is made to a paper by Mogab, Adams, and Flamm titled "PLASMA ETCHING OF Si AND SiO.sub.2 --THE EFFECT OF OXYGEN ADDITIONS TO CF.sub.4 PLASMAS" published in the July, 1978, issue of the Journal of Applied Physics on pages 3796-3803. J. A. Bondur and H. A. Clark published a paper titled "PLASMA ETCHING FOR SiO.sub.2 PROFILE CONTROL" in the April, 1980, issue of Solid State Technology on pages 122-128. These papers show that photoresists can be used to locally etch holes in the oxide that overlies a silicon substrate so that metal contacts can be applied to the silicon. Since the oxide can be made to etch faster than the silicon, the plasma etch process has achieved considerable significance in the semiconductor device processing art.
In modern semiconductor device technology sizes have been scaled down and it is often desirable to create micron sized contacts to the silicon. This is close to the limit attainable using present photolithographic techniques because a micron is on the order of the wavelength of visible light. Actually such a contact diameter is about the same size as the thickness of the surface passivation layer which is usually predominantly SiO.sub.2. It has been found that it is increasingly difficult to properly etch smaller contact holes. At the micron size it is found that on some parts of a semiconductor wafer the contacts will penetrate the oxide down to the silicon. However, in other areas extending etching must be employed to ensure proper penetration. In this case the contacts that were sufficiently etched originally will now be overetched and will either penetrate into the silicon or will be oversized due to lateral oxide etching. In any event a reliable contact formation becomes difficult to achieve over the entire surface of a wafer.